Energy savings is an important issue in many organisations today, both for economical and environmental reasons. Many organisations have set up goals concerning how much the direct and indirect CO2 emissions should be reduced in the years to come. For organisations supplying network elements for telecommunication systems, a large part of the indirect carbon-related emissions take place in the user phase.
Further, it is desired to reduce the interference in communication systems, which may be addressed e.g. by reducing transmission power.
For example, wireless network nodes such as e.g. radio base stations or relays, consume a considerable amount of energy when transmitting information to other network elements. The Radio Unit (RU) of a base station, which performs the transmissions, uses a considerable part of the total consumed power of the base station. A RU comprises one or more transmitters, each of which in turn comprises one or more Power Amplifiers (PAs). A PA in an average base station has an output power of approximately 20 W and an efficiency level of around 20%, which means that approximately 100 W are needed in order to obtain an output power of 20 W from a PA. Considering the amount of network nodes transmitting 24 h/day, 365 days/year in a communication system, it is apparent that even small improvements in the transmission power efficiency of a network node may have a notable impact on the total amount of consumed power in a large perspective.
It is comprehensible that a great amount of energy is consumed in a network node during the busy hours of the day, when the network or cell has a high load level. However, it is perhaps not as evident that as much as up to 23.8% of the maximum power consumed in a network node during the busy hours, also is consumed during periods when the network or cell load is zero. The reason for this is that control information is transmitted by a network node also when there is no traffic load in the network or cell. The table in FIG. 16, shows an example of the percentage of user data and control signals, respectively, in an LTE (Long Term Evolution) radio frame at different traffic loads. In the example, an LTE cell with 20 MHz system bandwidth, 1 TX (transmission) antenna and 3 PDCCH (Physical Downlink Control Channel) symbols per subframe, is assumed. It can be seen in the table in FIG. 16 that at maximal traffic load, the control signalling represents 40% of the total amount of symbols per radio frame. At zero traffic load, the control signalling represents 23.8% of the total amount of symbols per radio frame, which implies that 23.8% of the total transmission power used when transmitting a subframe at maximum traffic load in the LTE network node is consumed at zero traffic load, as stated earlier.
Thus, control signalling consumes a considerable amount of energy also during periods of low to moderate traffic load. A considerable part of the amount of cells in a network will have a low to moderate traffic load during a considerable part of the time. Further, the relatively high transmission power levels during low to moderate traffic loads contribute to the interference level in the network. This leaves room for further improvements.